The T Antigens (large, small and now perhaps middle) of simian virus 40 are products of the viral early locus. The early locus encodes the information controlling induction of the DNA-synthetic machinery of the cell, initiation of replication at the site of origin on the SV40 DNA, and initiation of oncogenic transformation. The purpose of this project is to study the physical chemical, protein chemical and enzymological characteristics of the T antigen especially large T Ag. The cause of the heterogeneity of T antigen upon sedimentation in sucrose gradients and chromatography on either DNA or Blue Sepharose is to be investigated. We plan to determine whether the new 48K-55K "middle T Ag" species are virus coded. We plan to study the self association of large T Ag and its enzymatic activity.